As the semiconductor industry continues to increase circuit complexity and density by reduction of process node geometries, operating signal frequencies continue to increase. It is now possible to obtain semiconductors that operate well into the millimeter wave region of radio spectrum (30 GHz to 300 GHz). Traditionally, the types of semiconductors used have been in the category of “III-V” types, indicating that the semiconductor compounds have been derived from periodic table elements in the third and fifth columns, such as gallium arsenide (GaAs) and indium phosphide (InP). In recent years, less expensive semiconductor processes that arise from column IV of the periodic table, such as silicon (Si) and germanium (Ge) have been produced in silicon CMOS (complementary metal oxide semiconductor) and silicon germanium (SiGe) compounds. The result has been to extend the operating frequency of low-cost silicon semiconductors well into the 60 to 80 GHz range of frequencies. The availability of low-cost semiconductor technology has put pressure on millimeter wave manufacturers to bring the overall costs down for the electromechanical support mechanisms that enable these semiconductor devices.
Commercial waveguide structures enable low-loss energy transfer at millimeter wave frequencies, with the additional benefit of standardization of size and mechanical coupling flange designs. The standardized sizes and coupling flanges enable interoperability between different devices and different manufactures, providing maximal flexibility for millimeter wave system design.
Traditional methods for interfacing semiconductor devices within a mechanical waveguide have been to either provide a split-cavity type of assembly with expensive precision machining requirements or to couple energy from an orthogonal planar printed circuit launch probe with associated lossy energy transfer. With new semiconductor designs providing balanced transmission line outputs, there has been no straightforward electromechanical method for coupling millimeter wave energy from the balanced outputs directly to a waveguide without added circuitry, such as a balun transformer, which also exhibits excessive losses as the frequency range of operation increases.
The prior art methods for coupling energy into and out of semiconductor devices, as set forth above, can be divided into two categories. The first category involves the use of split-cavity metallic structures that allow the semiconductor chip to be placed into one of the cavities, with the other half of the cavity then brought together with the first half in a precision fit. The typical precision required for the internal dimensions of a millimeter wave waveguide is on the order of ±0.001″ (0.025 mm). Obtaining this level of precision in the construction of both upper and lower cavity halves of a split cavity metallic structure through machining while maintaining registration alignment for such an assembly is expensive.
The second category used for coupling energy in and out of semiconductor devices is to provide a printed circuit board with a stub or paddle energy launch. The stub or paddle launch is orthogonal to the waveguide cavity, also requiring a split-cavity type of assembly method, creating additional expense.
In each case, a custom, highly precision machining process is required to maintain the internal waveguide dimensional requirements. Some cost reduction can be afforded through a casting process, but secondary machining operations are still necessary to realize the precision needed.
The above prior methods also are designed for single-ended circuit configurations only. It is necessary, however, to provide low-cost and efficient coupling methods for both single-ended and differential circuits. Millimeter wave semiconductor circuit designs often make use of differential amplifier and output stage configurations to enable high gain and power efficiencies.